A transmission system according to the preamble is known from the book Information Theory and Reliable Communication" by Robert G. Gallager, Chapter 3, Coding for Discrete Sources, pp. 38-70, ISBN 471 29048 3.
Transmission systems according to the preamble are used for transmitting sequences of symbols having unequal probabilities. Such sequences can be transmitted more efficiently by using short encoded sequences for input sequences having a high probability, and using longer encoded sequences for input sequences having a lower probability.
Examples of variable length codes are the well known Morse code and the Huffman code. In general, the use of variable length coding results in a decreased number of symbols that has to be transmitted for transporting a particular sequence of input symbols. But it is also possible that the number of symbols required to transport a particular sequence of input symbols is substantially larger than the uncoded message. This can happen if the sequence of input symbols comprises symbols having a low probability. In such a case practical problems can arise such as overflow of buffers or the impossibility to put the encoded message in a frame having only a limited space available for it.